1. Field of the Invention
This invention relates to a method and appartus for propagating potential inversion wells, together with minority carriers which may be contained therein, along the surface of a semiconductor, and to practical applications for such improved propagation techniques.
2. Description of the Prior Art
Recent research efforts have lead to the discovery that electrically isolated potential inversion wells may be established in the surface of a semiconductor, filled with minority carriers, and moved from one spot to another on the semiconductor by a surface field. See, for example, the articles in the Bell Systems Technical Journal, Apr. 1970 by Boyle and Smith at page 587 and by Amelio et al at page 593, the article in Electronics, Volume 43, No. 10, (1970) at page 112 by Altman, and the article in Scientific American, Feb. 1974, by Amelio at page 23.
The well movement or propagation technique disclosed in the above articles is a step-by-step process involving the sequential energization of digital electrodes. This is generally illustrated in FIGS. 1A - 1C, which show a p-type semiconductor 10 having an upper layer of insulation 12 on which discrete electrodes 14 are disposed, sequentially connected in three separate groups. If a positive voltage V.sub.2 &gt;V.sub.1 is applied to the lower connecting line, as in FIG. 1A, potential inversion wells 16 will be established under the first group of electrodes. Minority carriers, in this case free electrons, may be injected into selected wells, as shown in the first and third wells for example. In effect, the V.sub.2 potential repels any holes or "positive charges" adjacent the upper surface of the semiconductor 10 to thereby form a depletion region as generally indicated by contour line 18.
The movement of the potential wells 16 may be implemented by sequencing the voltage on the three connecting lines, as shown in FIGS. 1B and 1C. That is, if a voltage V.sub.3 &gt; V.sub. 2 is applied to the middle connecting line, deeper wells are established adjacent the wells created by the V.sub.2 potential, and the minority carriers are attracted into the deeper wells by the stronger V.sub.3 potential. When V.sub.3 and V.sub.2 are reduced to the V.sub.2 and V.sub.1 levels, respectively, the remaining wells, which have been moved over one electrode position, retain any minority carriers present.
This method of well propagation necessarily produces bumpy wells, however, particularly during the voltage sequencing transitions, and the attendant jerky well motion results in a considerable loss of minority carriers during propagation which severely limits the stage length of any device incorporating this technology.
A further problem is that due to thermal generation and junction leakage a charge domain will develop within one second of the application of a high field to the semiconductor surface, regardless of the initial presence or absence of a domain. In other words, if a voltage is applied to an electrode group to thereby establish potential wells in the semiconductor surface beneath the electrodes, the free minority carriers in the semiconductor will migrate to the wells to form charge domains within one second. To circumvent this problem the duration of the propagating potentials must be considerably less than one second, which requires very precise and accurate timing circuitry.